Method and apparatus for absolute interferometry using a measurement beam and a reference beam having parallel wave fronts and sharing a single beam path

Abstract

A method and apparatus are disclosed for implementing absolute interferometry with radiation generated by diode laser whereby relative or absolute distance measurement can be performed without need of a reference interferometer, with only a single detector, and with high precision determination of the length of the measurement distance. The method comprises modulating the radiation generated by the diode laser in such a way that the diode laser is frequency-modulated at at least one discrete frequency, which then is detected with a known phase-sensitive detection method. Due to the modulation of the original radiation of the diode laser, a second laser beam is in effect produced, with the result that a single beam path includes a measurement beam and a reference beam. Each of these beams, i.e. the measurement beam and the reference beam, is at a different frequency, but the two have parallel wave fronts, pass along exactly the same optical path in the unitary beam, and interfere at the single detector. The length of the measurement distance is then found by a determination of phase shift or determination of the phase change of the modulation signal. Alternatively, the modulation frequency of the diode laser can be changed until the detector shows that a predetermined phase shift between the original first beam of the diode laser and the second, modulation-produced beam, with altered frequency, exists or has been passed through. By determining the modulation frequency at which the predetermined phase shift is reached, and taking the difference between this and the initial modulation frequency, the absolute length of the measurement distance can be found.

What is claimed is: 
     
       1. A method of absolute interometry for determining the length of a measurement distance wherein a first beam is generated by a diode laser, and the measurement distance is the distance between the diode laser and a reflector as determined by means of a detector, and comprising the steps of high-frequency modulating the first beam ν1 of the diode laser in such a way that in the frequency range separated from a carrier frequency by an amount corresponding to a modulation frequency νm at least one resulting side band νs is produced with νs=ν1+νm, so that the diode laser in addition to said first beam with the carrier frequency simultaneously generates a second beam with side band frequency but parallel wave fronts,   said first and second beams each pass along the same optical path within a single optical beam, and   the interference of said first and second beam with one another is measured at the detector as a corresponding detector signal; and   measuring the phase shift Δφ, comprising the phase difference between said detector signal and modulation reference signal, as a measure of the change in the length comprising the measurement distance.   
     
     
       2. A method as claimed in claim 1, wherein the change in the length comprising the measurement distance ΔL is determined as follows:   ΔL=Δφ·c/ (2·νm),     where c is the velocity of light in the medium.   
     
     
       3. A method as claimed in claim 1, wherein for relative measurements the direction of the change in the length comprising the measurement distance is determined by electronically splitting the detector signal into equal parts, sending the parts to at least two phase-sensitive detectors, and phase shifting the reference signals of the phase-sensitive detectors by one of 90°, 180° and 270° so as to produce one in-phase and one quadrature signal by which the direction of the change in length can be determined. 
     
     
       4. A method as claimed in claim 1, wherein the radiation of the diode laser is modulated with two different frequencies and the resulting difference frequency is detected. 
     
     
       5. A method as claimed in claim 1, wherein the intensity of the measured signals is normalized with respect to the radiation power at the detector. 
     
     
       6. A method as claimed in claim 1, wherein a movement velocity between the diode laser and the reflector is determined from the change in length of the measurement distance between the diode laser and the reflector per unit time. 
     
     
       7. A method as claimed in claim 1, wherein a change in distance between the diode laser and the detector is determined. 
     
     
       8. A method of absolute interferometry for determining the length of a measurement distance wherein radiation is generated by a diode laser, and the measurement distance comprises the absolute distance between the diode laser and a reflector as determined by means of a detector, and comprising the steps of modulating the radiation of the diode laser whilst simultaneously altering the modulation frequency from ν1 to ν2 until the detector shows a phase shift of 2πbetween an original first beam of radiation from the diode laser and a second beam of radiation produced by the modulation, the first and second beams of radiation having different frequencies but parallel wave fronts which travel along the same optical path;   measuring the modulation frequency ν2 at which the phase shift of 2πis reached and subtracting therefrom the initial modulation frequency ν1 to obtain a frequency change Δν in accordance with the equation Δν=ν2-ν1; and   determining the absolute measurement distance L by calculation in accordance with the equation   L=c/(2·Δν),        where c is a constant equal to the velocity of light in the medium.   
     
     
       9. Apparatus for absolute interferometry for determining the length of a measurement distance wherein radiation is generated by a diode laser, comprising a diode laser emitting radiation at a carrier frequency;   control means to operate the diode laser;   a radiation detector;   a modulator for high-frequency modulation of the radiation of the diode laser to produce radiation at a side-band frequency, the carrier frequency radiation and the modulated radiation together travelling over the same measurement distance to arrive at the detector where they interfere; and   a phase-sensitive detector with a first input connected to an output of the detector, a second reference input supplied with the modulation reference frequency of the modulator, and an output which supplies a signal representing a phase difference which is correlated with the length of the measurement distance.